Electronic Structure of O-vacancy in High-k Dielectrics and Oxide Semiconductors
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Electronic Structure of O-vacancy in High-k Dielectrics and Oxide Semiconductors Kee Joo Chang1, Byungki Ryu1, Hyeon-Kyun Noh1, Junhyeok Bang2, and Eun-Ae Choi3 1 Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 305701, Korea 2 Department of Physics, Rensselaer Polytechnic Institute, New York, USA 3 CAE TEAM, Memory Division, Samsung El. Co., Hwasung, Korea
ABSTRACT First-principles density functional calculations are performed to investigate the electronic properties of O-vacancy defects in high-k HfO2, Si/HfO2 interface, and amorphous oxide semiconductors. The role of O-vacancy in device performance is discussed by comparing the results of the GGA, hybrid density functional, and quasiparticle energy calculations. INTRODUCTION The technology roadmap reflects that advances in Si-based complementary metal-oxide semiconductor (CMOS) technology will reach the device scale of sub-0.1 ȝm [1,2]. As the thickness of insulating gate oxide is in the range of 1−2 nm, defects inside oxide and at interface will significantly modify the electrical properties of devices. The use of ultra-thin SiO2 gate dielectrics leads to a number of problems, such as high leakage current, reduced driving current, reliability degradation, and B penetration [3,4]. For a replacement of SiO2 gate oxide, HfO2 has been considered as a promising high-k dielectric material, because gate leakage current can be reduced by keeping the same effective oxide thickness [5]. However, this material suffers from a high density of defects, especially O-vacancy, which cause degradation of devices, such as flat band voltage (Vfb) shifts, threshold voltage (Vth) instability, and low carrier mobility [6,7]. On the other hand, in Hf-silicate based devices, device performance was shown to be greatly improved, with the reduction of threshold voltage shift and instability [8]. The suppression of active trap centers was suggested to be responsible for the reduction of the Vth instability [9]. In HfO2-based devices, Vfb shifts were attributed to the Fermi level pinning effect which is caused by interfacial Hf-Si bonds [10] or O-vacancies [11]. In the O-vacancy defect model, the Fermi level pinning is accompanied with a charge transfer between Si electrodes and defects in the oxide, which induces the interface dipole. In theoretical calculations which rely on the localdensity functional approximation, defect energy levels cannot be accurately determined because the band gaps are underestimated. Several advanced calculations have been performed for Ovacancy defects in HfO2 using the screened exact exchange method, the weighted density approximation [12], the hybrid density functional [13-15], and the quasiparticle energy calculations [16]. Despite many studies, the calculated defect levels are quite diverse. Moreover, as the calculations were mostly done for bulk HfO2, the interface effect on the defect properties was excluded. Recently, theoretical calculations were performed for O-vacancy defects near
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Si/HfO2 interfaces, reporting th
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